Physics Unit Two

Forces: Resultant Forces

Whenever two objects interact, the forces they exert on each other are equal and opposite.

A number of forces acting at a point may be replaced by a single force that has the same effect on the motion as the original forces all acting together. This single force is called the Resultant Force.

A Resultant Force acting on an object may cause change in its state, rest or motion.

If the Resultant Force acting on a stationary object is:

Zero - The object will remain stationary

Not Zero- The object will accelerate in the direction of the Resultant Force

If the Resultant Force acting on a moving object is:

Zero - The object will continue to move at the same speed in the same direction

Not Zero - The object will accelerate in the direction of the resultant force

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Forces, Motion and Braking

The acceleration of an object is determined by the Resultant Force acting on the object and the mass of the object.

The gradient of a distance-time graph represents speed.

The Velocity of an object is its speed in a given direction.

The acceleration of an object is given by the equation: a=(v-u)/t

The gradient of a velocity-time graph is represents acceleration.

When a vehicle travels at a steady speed the resistive forces balance the driving force.

The greater the speed of a vehicle the greater the braking force needed to stop it in a certain distance.

The stopping distance of a vehicle is the sum of the distance the vehicle travels during the drivers reaction time (thinking distance) and the distance it travels under the braking force (braking distance).

A drivers reaction time can be affected by tiredness, drugs and alcohol.

When the brakes of a vehicle are applied, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the braking increase.

A vehicle's braking distance can be affected by adverse road, weather conditions and poor conditions of the vehicle.

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Forces, Terminal Velocity and Elasticity

The faster an object moves through a fluid the greater the frictional force that acts on it.

An object falling through a fluid will initially accelerate due to the force of gravity. Eventually the resultant force will be zero and the object will move at its terminal velocity (steady speed).

A force acting on an object may cause a change in shape of the object.

A force applied to an elastic object such as a spring will result in the object stretching and storing elastic potential energy.

For and object that is able too recover its original shape, elastic potential energy is stored in the object when work is done on the object to change its shape.

The extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded.

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Forces: Energy

When an object speeds up or slows down, it kinetic energy increases or decreases. The forces which cause the change in speed do so by doing work. The momentum of an object is the product of the objects mass and velocity.